Magnetic field sensing device

ABSTRACT

Two sensor units are formed from magnetoresistive material. Elements of the first sensor unit have a total anisotropy field in a first direction. Elements of the second sensor unit have a total anisotropy field in a second direction. An integral coil sets a direction of magnetization in the elements of the first and second sensor units. An output of the first sensor unit is representative of magnetic field components perpendicular to the first direction and an output of the second sensor is representative of magnetic field components perpendicular to the second direction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/085,858, filed May 27, 1998.

U.S. GOVERNMENT RIGHTS (IF ANY)

Not applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to magnetic field sensingdevices and specifically to magnetic field sensing devices capable ofsensing magnetic fields along two mutually perpendicular axes. Such twoaxis magnetic field sensors are required in many applications.Electronic compasses, magnetic field probes, and virtual reality are afew examples of applications where two axis magnetic field sensors areuseful.

2. Description of the Prior Art

In the past, two axis magnetic field sensors were typically constructedusing two single axis magnetic sensors. For example, U.S. Pat. No.5,247,278 describes a single axis magnetic field sensor including anintegral current strap for setting a direction of magnetization. Two ofthe sensing devices described in U.S. Pat. No. 5,247,278 can be used toform a two-axis magnetic field sensing device. For simplicity, two-axismagnetic field sensing devices will be referred to herein as an x-axissensor and a y-axis sensor, meaning that the two axes are perpendicular.In the past, the two single axis sensors could be housed in a singlepackage enclosure and oriented so that their sensitive directions wereperpendicular to each other. Alternatively, two single-axisindividually-packaged die could be mounted on a circuit board with thesensitive axis of each die perpendicular to the other die. There aredisadvantages to the use of two single axis die. One disadvantage ofthis approach is that it requires extra assembly effort either at thepackage level or at the board level. In addition, it is difficult tolocate the two single-axis die so that they are orthogonal to eachother. The best control on the orthogonality of the two single-axisparts in high volume manufacture may be on the order of ±1°, whichinduces the same level error on compass heading.

A magnetoresistive sensor capable of measuring a low magnetic fieldrequires that the magnetic moment be initially aligned in one direction,which usually is perpendicular to the sensitive direction of the sensor.With a uniform external magnetic field to initialize the alignment ofmagnetic moment, it is almost impossible to have an x-axis and a y-axissensor on a single chip. In addition, generally a magnetic film used formagnetoresistive sensors will have its own crystal easy axis which isdetermined by a magnetic field applied during the deposition of themagnetic film. Single axis sensors typically utilize this easy axis andinitially align the magnetic moment along it. Single axismagnetoresistive sensors usually have the crystal anisotropic field andthe shape anisotropic field in the same direction to guard againstmagnetic and thermal disturbances and to maintain a stable and low noisesensor output. The stability of a magnetoresistive sensor is determinedat least to some extent by how good it maintains a single magneticdomain state after the magnetic field for aligning or setting themagnetization is removed.

An integrated two-axis magnetoresistive sensor must have a sensitivedirection in an x-axis and a sensitive direction along a y-axis. Thismeans that at least one of the sensor's directions cannot be alignedwith the crystal easy axis direction. Therefore, consideration must begiven to how to deal with the crystal easy axis when attempting toconstruct a two-axis sensor on a single die, and how to initially alignthe magnetic moment in both an x direction and a y direction.

The advantage of a two-axis sensor on one die is that the orthogonalityof the two sensors is controlled by the photolithography method, whichhas accuracy in the range of about 0.01°.

Thus a need exists for an integrated two-axis magnetoresistive sensor.

SUMMARY OF THE INVENTION

The present invention solves these and other needs by providing atwo-axis integrated device for measuring magnetic fields including twosensor units formed from magnetoresistive material having a crystalanisotropy field direction. Elements of the first sensor unit have atotal anisotropy field in a first direction. Elements of the secondsensor unit have a total anisotropy field in a second direction which isperpendicular to the first direction. Means are provided for setting adirection of magnetization in the elements of the first and secondsensor units. An output of the first sensor unit is representative ofmagnetic field components perpendicular to the first direction and anoutput of the second sensor is representative of magnetic fieldcomponents perpendicular to the second direction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top plan view of a simplified integrated circuit layoutaccording to the teachings of the present invention.

FIG. 2 shows a diagrammatic representation of certain principlesaccording to the teachings of the present invention.

FIG. 3 shows certain additional details of the circuit layout of FIG. 1.

DETAILED DESCRIPTION

A device for sensing magnetic field components along two axes is shownin the drawings and generally designated 10. FIG. 1 shows an integratedcircuit layout for a magnetic field sensor in accordance with thepresent invention. An integrated circuit die 12 has formed thereon aconductor or strap 14 which extends from set or set reset pad 16 in aclockwise four sided spiral form and terminates in pad 18.Magnetoresistive elements are formed of elongated strips ofmagnetoresistive material. FIG. 1 shows for example an element 20consisting of magnetoresistive strips 22 and 24 and interconnect 26.Only two magnetoresistive strips per element are shown for simplicity,but it is understood that an actual element could include many morestrips. Elements 20, 28, 30, and 32 are shown connected in a firstWheatstone bridge arrangement with a power supply connection at Vcc1(X)and Vcc2(X) and an output voltage connection between Vout+(X) andVout−(X). The direction of sensitivity of the first Wheatstone bridge isshown by arrow 33 and this bridge acts as the x-axis sensor.

Elements 34, 36, 38 and 40 are shown connected as a second WheatstoneBridge arrangement with a power supply connection at Vcc1(Y) and Vcc2(Y)and an output voltage connection between Vout+(Y) and Vout−(Y). In FIG.1, separate power supply connections are shown, however they areconnected to one common power supply. The direction of sensitivity ofthe second Wheatstone bridge is shown by arrow 41 and this bridge actsas the Y-axis sensor. Conductive paths as shown in FIG. 1 connect oneend of each of elements 20 and 32 of the first Wheatstone bridge and oneend of elements 34 and 40 of the second Wheatstone bridge to ground pad42. Now that the basic construction of magnetic field sensing device 10has been disclosed, the operation of device 10 according to theteachings of the present invention can be set forth and appreciated.

In the two-axis device of the present invention, the X sensor and the Ysensor must have sensitive directions perpendicular to each other. Inthe specific embodiment of FIG. 1 the easy axis 44 of the crystalanisotropy field of the magnetoresistive material film is in onedirection, that is, it is either 0° or 180° for both the x-axis portionand the y-axis portion of the two sensor units formed in the material.The direction is determined by magnetic field direction during thedeposition and annealing of the magnetoresistive material. Otherembodiments of the present invention could use films that do not have acrystal anisotropy field.

The construction and operation of the two-axis magnetic field sensingdevice of the present invention will be explained by reference to theuse of elongated strips of a magnetoresistive material such as Permalloywhich are interconnected. Various other constructions are possible.

An elongated strip of magnetoresistive material may be considered tohave an associated crystal anisotropy field and a shape anisotropyfield. The total anisotropy field is the vector sum of the crystalanisotropy field and the shape anisotropy field. FIG. 2 shows, accordingto the principles of the present invention, the relationship of theanisotropy fields. FIG. 2 shows, for the x-sensor, the easy axis 46 ofthe crystal anisotropy field, the easy axis 48 of the shape anisotropyfield, and the easy axis 50 of the total anisotropy field. The easy axis48 of the shape anisotropy field is along the length of the elongatedstrip. By way of example and not by way of a limitation, the crystalanisotropy field may be about 3 Oersted (Oe), and the shape anisotropyfield about 25 Oe. In this example, the shape anisotropy field 48 isdisplaced from the crystal anisotropy field 46 by about 50° in order tocause the total anisotropy field 50 to be displaced from the crystalanisotropy field 46 by about 45°. FIG. 2 also shows for the y-sensor,the easy axis 52 of the crystal anisotropy field, the easy axis 54 ofthe shape anisotropy field, and the easy axis 56 of the total anisotropyfield. The easy axis 54 of the shape anisotropy field is along thelength of the elongated strip. Thus for each of the sensors, the totalanisotropy field is the vector sum of the crystal anisotropy field andthe shape anisotropy field. The sensitive direction of a sensor will bein a direction perpendicular to the total anisotropy field. In order toarrange two sensors on a single die or chip to be sensitive in twomutually perpendicular directions, the total anisotropy field of the twosensors must be perpendicular to each other. In the preferred embodimentof FIG. 1, device 10 is constructed with the x-axis sensor and they-axis sensor arranged symmetrically relative to the crystal easy axis44 of the magnetoresistive material. For the specific embodiment of FIG.1, the elements of the x-axis sensor, i.e., elements 20, 28, 30 and 32,are rotated counterclockwise from the crystal axis by about 50° and theelements of the y-axis sensor, i.e., elements 34, 36, 38 and 40, arerotated clockwise by about 50°.

FIG. 3 shows a magnetoresistive strip 60 of the type that could be usedto make up the bridge elements shown in FIG. 1. A conductive strip 62extends across strip 60 and makes an angle 64 of about 50° with strip60. This angle, of course, will depend on the relationship of thecrystal anisotropy field, shape anisotropy field and total anisotropyfield for the specific device.

In the traditional design of elements using elongated magnetoresistivestrips, the barberpoles have been located at plus or minus 45° to thestrips. According to the teachings of the present invention, the widthof the barberpoles, the gap between barberpoles, and orientationrelative to the magnetoresistive strip need to be optimized to providean average current flow in the magnetoresistive material that is withinabout ±45° to the easy axis of total anisotropy fields in both X and Ysensors. In the present invention, within the borders of minimum widthand maximum gap, the current flow direction mainly is determined by thebarberpole orientations. The barberpole orientations for a specificelement are either along the crystal anisotropy field direction orperpendicular to the crystal anisotropy field direction, depending onthe position of the element in the Wheatstone bridge.

Now that the construction and operation of device 10 have beendescribed, additional advantages can be set forth and appreciated. Inthe embodiment of FIG. 1, the first sensor unit and the second sensorunit are located in a first plane, and a coil 14 is located in a secondplane. A single coil 14 may be used as a set coil or set/reset coil forboth the X sensor and the Y sensor. Coil 14 provides alignment along thetotal anisotropic field direction for both the X sensor and the Ysensor. By passing a current through coil 14, a magnetic field isprovided which is used to generate or set a single domain state in eachsensor element before using device 10 to make a measurement or reading.The field provided by the current should be large enough to set themagnetization in a single direction. The current may be used to simplyset the magnetization prior to a reading. The current may also beapplied in one direction prior to taking a first reading. The currentmay then be applied in the opposite direction before taking a secondreading in what is referred to as a set/reset application. The use of asingle coil permits a reduced size for the die and also results inreduced power consumption.

The operation of device 10 can be explained by using an arbitraryreference to certain directions. The elements of the x-axis sensor ofFIG. 1 are rotated counterclockwise from crystal axis 44. For example,element 20 is rotated about 50 degrees to cause its total anisotropyfield to be displaced from the crystal anisotropy field 44 by about 45degrees and to be in a first direction. The easy axis of the shapeanisotropy field of element 20 is along the length of element 20. Theelements of the y-axis sensor of FIG. 1 are rotated clockwise fromcrystal easy axis 44. For example element 34 is rotated about 50 degreesto cause its total anisotropy field to be displaced from the crystalanisotropy field 44 by about 45 degrees and to be in a second direction.The easy axis of the shape anisotropy field of element 34 is along thelength of element 34. The second direction corresponds with the totalanisotropy field of element 34 and is perpendicular to the totalanisotropy field of element 20. Passing a current through coil 14 willresult in a magnetic field, which, for example, at element 20 of thex-axis sensor will be perpendicular to coil 14 at the location ofelement 20, and in the first direction. Similarly, the same currentthrough coil 14 will result in a magnetic field, which, for example, atelement 34 of the y-axis sensor will be perpendicular to coil 14 at thelocation of element 34, and in the second direction.

The present invention has been described with reference to the specificembodiment of FIG. 1; however, other embodiments will be apparent. Forexample, with regard to the magnetoresistive material, the thin filmsused for the magnetoresistive sensor are deposited on substrates.Different substrate underlayer and different deposition conditionsresult in either textured polycrystal thin film or random distributedpolycrystal thin films.

Permalloy films grown on silicon substrates in the presence of amagnetic field usually have magnetic preferred orientation. That is, itis a textured film and has an effective crystal anisotropy field.However, with carefully chosen substrates, the film deposited in theabsence of a magnetic field could be random distributed, and without anymagnetic preferred orientation, which means no effective crystalanisotropy field existing in the film.

Alternative embodiments of device 10 may use a random distributed filmwith no effective crystal anisotropy field. In this alternativeembodiment, the total anisotropy field would include only the shapeanisotropy field component and would be along the length of amagnetoresistive element. In this embodiment, the angle of themagnetoresistive elements with the set reset strap would be 90°, ratherthan the 95° angle shown in FIG. 1.

Spatial relationships other than those shown in FIG. 1 for bridgeelements 34, 36, 38 and 40, and the set-reset strap 14 can be used. Forexample, the four elements of one bridge could be arranged so that themagnetization was set in the same direction in four elements. Aset-reset strap or coil could have a meander form or a serpentine form,or other forms, rather than the spiral form of FIG. 1. Two coils couldbe used rather than the single coil of FIG. 1.

A single magnetoresistive strip could form a leg of a Wheatstone bridge,rather than the multiple strips of magnetoresistive material shown inFIG. 1.

Magnetoresistive elements could be devised with different barberpoleorientations for different portions of a single leg of a Wheatstonebridge, with the set-reset current flowing in opposite directions at thedifferent portions of the single leg.

In addition, spatial relationships of elements, arrangements ofbarberpoles, forms of a set-reset strap and other variations notspecifically described herein can be devised.

Thus since the invention disclosed herein may be embodied in otherspecific forms without departing from the spirit or generalcharacteristics thereof, some of which forms have been indicated, theembodiments described herein are to be considered in all respectsillustrative and not restrictive. The scope of the invention is to beindicated by the appended claims, rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

1. An integrated device for measuring magnetic fields comprising: afirst sensor unit and a second sensor unit comprising magnetoresistivematerial and formed on a single substrate; said first sensor unit havingan output related to magnetic field components in a first direction;said second sensor unit having an output related to magnetic fieldcomponents in a direction substantially perpendicular to said firstdirection; and said integrated device being independent of a biasingmagnet.
 2. The integrated device of claim 1 wherein said singlesubstrate is a semiconductor substrate.
 3. The integrated device ofclaim 1 further comprising means for setting a direction ofmagnetization in at least a portion of said first sensor unit and forsetting a direction of magnetization in at least a portion of saidsecond sensor unit.
 4. The integrated device of claim 3 wherein saiddirection of magnetization set in said portion of said first sensingunit and said direction of magnetization set in said portion of saidsecond sensor unit are substantially perpendicular.
 5. The integrateddevice of claim 4 wherein said first sensor unit and said second sensorunit are formed in a common plane and said means for setting a directionof magnetization is a conductor located in a plane spaced from saidcommon plane.
 6. The integrated device of claim 5 wherein said firstsensor unit and said second sensor unit each comprise four bridgeelements.
 7. The integrated device of claim 6 further comprising biasingmeans selected to change an angle between a current in said bridgeelements and a direction of magnetization in said bridge elements. 8.The integrated device of claim 7 wherein said biasing means comprisesangled strips of a conductive material extending across at least aportion of said bridge elements.
 9. The integrated device of claim 8wherein said bridge elements are non perpendicular to said conductor andsaid angled strips form an angle about 45 degrees with said coil. 10.The integrated device of claim 8 wherein said bridge elements areperpendicular to said conductor.
 11. An integrated device for measuringmagnetic fields comprising: a first sensor unit and a second sensor unitformed in a common plane on a single substrate, said first sensor unitand said second sensor unit comprising magnetoresistive material; saidfirst sensor unit having a first output and said second sensor unithaving a second output; said first sensor unit comprising at least onesensing element having a total anisotropy field in a first direction;said second sensor unit comprising at least one sensing element having atotal anisotropy field in a second direction with said second-directionbeing substantially perpendicular to said first direction; a conductorlocated in a plane spaced from said common plane for setting a directionof magnetization in at least a portion of said at least one sensingelement of said first sensor unit in said first direction and setting adirection of magnetization in at least a portion of said at least onesensing element of said second sensor unit in said second direction; andwherein said first output is representative of magnetic field componentsperpendicular to said first direction and said second output isrepresentative of magnetic field components perpendicular to said seconddirection.
 12. The integrated device of claim 11 wherein said substrateis a semiconductor substrate.
 13. An integrated device for measuringmagnetic fields comprising: a first sensor unit and a second sensor unitformed in a single plane on a single substrate, said first sensor unitand said second sensor unit being formed from magnetoresistive materialhaving a crystal anisotropy field having a crystal anisotropy easy axis;said first sensor unit comprising a first plurality of elongatedmagnetoresistive sensing elements connected into a bridge arrangementhaving a first output with each of said sensing elements having a shapeanisotropy field having a shape anisotropy easy axis along a length ofsaid element, and a total anisotropy field having a total anisotropyeasy axis and each of said sensing elements being oriented with saidtotal anisotropy easy axis rotated through a first angle in acounterclockwise direction away from said crystal anisotropy easy axis;said second sensor unit comprising a second plurality of elongatedmagnetoresistive sensing elements connected into a bridge arrangementhaving a second output with each of said sensing elements having a shapeanisotropy field having a shape anisotropy easy axis along a length ofsaid element and a total anisotropy easy axis with each of said sensingelements being oriented with said total anisotropy easy axis rotatedthrough said first angle in a clockwise direction away from said crystalanisotropy easy axis; a conductor located in a plane spaced from saidcommon plane for setting a direction of magnetization in said sensingelements of said first sensor unit and for setting a direction ofmagnetization in said sensing elements of said second sensor unit; andsaid first angle having a value selected so that said first output isrepresentative of first magnetic field components perpendicular to saiddirection of magnetization set in said sensing elements of said firstsensor unit and said second output is representative of second magneticfield components perpendicular to said direction of magnetization set insaid sensing elements of said second sensor unit and said first magneticfield components are perpendicular to said second magnetic fieldcomponents.
 14. The integrated device of claim 13 wherein said crystalanisotropy field has a value sufficiently low when compared to saidshape anisotropy field that said shape anisotropy easy axis of saidsensing elements of said first sensor unit is oriented substantiallyperpendicular to said shape anisotropy easy axis of said sensingelements of said second sensor unit.
 15. The integrated device of claim14 wherein said conductor sets a direction of magnetization in saidsensing elements of said first sensor unit along a direction of saidshape anisotropy field and in said sensing elements of said secondsensor unit along a direction of said shape anisotropy field.
 16. Theintegrated device of claim 13 wherein said conductor sets a direction ofmagnetization in said sensing elements of said first sensor unit along adirection of said total anisotropy field and sets a direction ofmagnetization in said sensing elements of said second sensor unit alonga direction of said total anisotropy field.
 17. The integrated device ofclaim 13 wherein said single substrate is a semiconductor substrate.